I too have gone the other way around. I was working with the ATmega series for a long time, then I got an arduino uno as a gift and have used it for quite a few things. The one thing I have found the Arduino most useful in is testing new chips. When you are working with an unfamiliar chip (unfamiliar to you or new chip in general) you can generally write some code for the arduino and be sure that the arduino is not the problem. This is one of the most annoying things in hardware hacking, when testing you are not sure where the point of failure is. Having an arduino, or similar boards for that matter, eliminates the lurking suspicions that maybe the test gears is the problem instead of the chip being tested.